upstream: drop stats from original upstream clusters missing zone

[mpoegel]

Commit Message: drop stats from original upstream clusters missing zone
Additional Description: Original destination and dynamic forward proxy clusters create hosts with empty locality zones, producing stats with double dots (e.g., cluster.foo.zone.bar..upstream_rq_200). StatNameDynamicStorage("") produces a non-empty StatName that bypasses the empty zone guard in codes.cc, unlike StatNameStorage("") which correctly produces an empty one.
This PR fixes localityZoneStatName() to return a truly empty StatName when the locality zone is empty. This then matches what I believe is the intended logic in codes.cc to skip stats that are missing either zone.
Risk Level: Low
Testing: Added a new unit test
Docs Changes: N/A
Release Notes: N/A
Platform Specific Features: N/A
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[repokitteh-read-only]

Hi @mpoegel, welcome and thank you for your contribution.

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🐱

Caused by: #45215 was opened by mpoegel.

see: more, trace.